IB/mthca: Replace dma_sync_single() use with proper functions

dma_sync_single() is deprecated now, and the use in mthca is wrong:
there should be a dma_sync_single_for_cpu() before touching the memory
from the CPU, and a dma_sync_single_for_device() afterwards.  Fix
this, prompted by a kick in the pants from a patch from FUJITA
Tomonori <fujita.tomonori@lab.ntt.co.jp>.

Signed-off-by: Roland Dreier <rolandd@cisco.com>